Ink jet recording is known as an excellent recording method suitable for a comparatively wide range of recording media, and recording apparatuses, recording methods, and recording media based on the ink jet approach have been widely researched and developed.
In recent years, performance improvement of printers, inks, and recording media made it possible to obtain easily a high quality on par with that of halide photography. In this respect, the advancement of recording media is remarkable, and presently available recording media that were developed to realize high gloss, while increasing the absorption rate of ink and the amount of absorbed ink are at a level surpassing that of the conventional commercial printing in terms of gloss and quality feel. Such recording media can be generally classified into swellable and porous, but in recent years porous recording media are mainly used because they excel in ink drying rate.
Examples of such porous recording media are disclosed in Patent Literature 1 and Patent Literature 2. In these media, an ink absorption layer containing silica and alumina hydrate and having pores for taking in the ink is provided on a support body, and a porous glossy layer containing colloidal silica is provided, if necessary, on the ink absorption layer. With such configuration, excellent ink absorption ability and very fine output are obtained. Therefore, such recording media are suitable for photographic output applications for consumers. However, because starting materials for the recording media of this type are very expensive and the manufacturing process is complex, the media are much more expensive that the general commercial-grade and publication-grade coated paper. As a result, despite high quality of images, such media have not yet been widely used in the field of commercial printing where low cost and high output are required, e.g. for flyers, catalogs, and pamphlets. Accordingly, a great deal of efforts went heretofore into cost reduction of recording media, but because a filler constituting an ink absorbing layer of recording media is required to use a material that can maintain high transparency of ink absorbing layer and has high oil absorption quantity (specific surface area), it is necessary to use a large amount of a special expensive filler such as silica, alumina hydroxide, and colloidal silica, thereby greatly hindering cost reduction.
Further, where inexpensively produced coated paper in which a large amount of inexpensive fillers having high hiding ability and a comparatively low oil absorption quantity, such as calcium carbonate and kaolin, is added to a coating layer material, as the commercial-grade and publication-grade coated paper, is used as a recoding medium, the images blur significantly and density is not developed. This result can be explained as follows. Because commercial-grade coated paper is not designed to be capable of absorbing a large quantity of ink within a short period, as ink jet paper, ink absorption is insufficient and blurring occurs, and even if the ink permeates into the coating layer, the colorant is hidden by a filler with a high hiding ability such as kaolin present in the coating layer. Further, in commercial-grade and publication grade coated paper using fillers with high hiding ability, when printing is performed in an ink jet printer using dye inks, density is demonstrated only by the colorant present close to the surface layer, even if the amount of ink is increased. Therefore, an image that as a whole has low density and no contrast is obtained. Accordingly, commercial-grade and publication grade coated paper has not been suitable for ink jet recording.
In recent years, ink-jet inks using pigments rather than dyes as colorants have attracted much attention. Because pigments are insoluble in water, a dispersion is usually used in which pigment fine particles are dispersed in a solvent. Due to safety considerations, pigment inks for ink jet printing are mainly aqueous dispersions. Aggregation or precipitation of pigment particles in aqueous pigment inks generally occurs easier than in the dye inks. Accordingly, dispersion conditions and additives have to be appropriately selected to obtain long-term storage life on par with that of dye inks. Another drawback is that dispersion stabilizers cause cohesion and they are difficult to use with thermal heads. Moreover, a color range of colorants is inferior to that of dyes. On the other hand, pigment inks are very attractive because of high density and excellent stability in storage and resistance to water after recording. In ink jet printer using such pigment inks, ink colorants are sometimes close to colorants of general inks for commercial printing, and the appearance of prints can be brought close to commercial prints, but where printing is actually performed with the conventional ink jet printers using pigment inks on commercial-grade and publication-grade coated paper, ink absorption is insufficient, the images blur, pigments are not fixed completely after drying, and gloss cannot be obtained, that is, such printing is presently adapted only to printing on media with a high ink absorption ability such as usual paper and special paper for ink jet printing, as with the conventional dye inks. This is because the concept and design relating to ink jet image printing is the same as in the case of using dye inks, and pigments are used as if they are dyes with a high light resistance. In other words, specific features of pigment inks are not taken into account at all.
Recording media adapted to pigment inks have also been widely researched and developed, but the point of development is how to promote image gloss, while accelerating ink absorption, as described in Patent Literature 3, and the research of methods for developing gloss, while increasing the porosity of recording media surface is still pursued along the line of conventional technology using expensive filler materials.
In order to resolve the above-mentioned problems, Patent Literature 4 suggests a low-cost method for recording images on commercial-grade paper by combining a pigment ink with high permeability and a recording medium that, by contrast with the conventional media, has low ink absorption ability. With this method, only a solvent (water or an organic solvent) forming the ink is selectively caused to infiltrate into the substrate by recording using a small amount of pigment ink with ultrahigh permeability on a recording medium having provided thereon a coating layer for inhibiting ink absorption (permeation) so that the pigment contained in the ink does not infiltrate intensively into the medium, and the pigment contained in the ink can be caused to remain on the medium surface with good efficiency, without using any special material such as cation fixing agent. As a result, both a sufficient density and drying ability can be realized with a small amount of ink. Moreover, because the pigment contained in the ink, is caused to remain effectively on the medium surface, high transparency of the layer that was a necessary function in the conventional recording media becomes unnecessary. Therefore, the degree of freedom relating to material configuration of the coating layer can be greatly increased. By applying this method, it is possible to perform ink jet recording even on paper with low ink absorption ability such as commercial-grade or publication-grade paper.
When commercial printing and publication printing is performed, it is often necessary to output several hundreds to several thousands copies at one time even in small-lot printing, and the printer is required to produce stable images without image loss in a continuous mode. When an ink jet printer is used for such applications, image streaks occur because dried ink clogs the nozzles and the discharge trajectory is curved by the dried ink fixed to the periphery of the nozzle. Various means have been used to prevent this phenomenon, but the most effective among them, for both the dye inks and the pigment inks, is the addition of a wetting agent with a high boiling point to the ink with the object of obtaining a mild drying ability of the ink.
However, although the method described in Patent Literature 4 is very effective for recording on paper with poor ink absorption, when a recording medium with extremely poor ink absorption, such as commercial-grade ink, is combined with an ink additionally containing a wetting agent with a high boiling point for preventing the printer head from drying (nozzle clogging), the drying ability of image does not change significantly, but an extremely long time is required for “fixing”, that is to reach a state, in which the dried image is not blurred even by rubbing. This is because the colorant remaining in the surface layer contains microamounts of the wetting agent and remains in a wetted state for a long time. This phenomenon is similar to that observed when offset printing ink using soybean oil requires a long time for fixing after printing. In terms of such fixing delay, pigment inks appear to be especially disadvantageous by comparison with dye inks. With the dry-type inks that are presently mainly used as ink-jet inks, the colorant itself easily diffuses into a medium. As a result, a wetting agent (in most cases, a solvent with a high boiling point) also diffuses within a relatively short period. As a result, when recording is performed by using commercial-grade paper in a printer using ink that contains a wetting agent with a high boiling point in order to improve reliability against image streaks on the basis of the method described in Patent Literature 4 that is designed for the use of pigment inks, the method demonstrates poor convenience in most cases. Thus, the fixing time is long and the method cannot be adapted to distribution, e.g. of flyers and catalogs, immediately after recording.
Various attempts have been made to improve drying ability and fixing ability of ink prints immediately after recording. In particular, when application of ink jet technology to consumer-oriented products was started, thermal drying was considered to be effective for improving drying ability of inks and recording media. For example, Patent Literature 5 suggests that the image be heated and fixed after recording is performed using colored resin particles dispersed in the form of fine particles. However, this suggestion is mainly aimed at the reduction of blurring in paper that runs easily such as recycled paper and copy paper, and the object of fixing is also paper with comparatively rapid absorption of ink such as recycled paper and copy paper.
Further, Patent Literature 6 describes using a heating roller as a drying aid means for ink prints, but the attention is mainly focused on dye-based oily inks, and application to aqueous pigment inks and commercial-grade paper is not disclosed.
Patent Literature 7 suggests a method for heating a recording media on which ink jet recording has been completed from a rear surface in order to enhance drying and thereby improve fixing ability. This document describes that drying and fixing are enabled by heating from the rear surface. However, with the suggested method, because the image is not heated directly, thermal efficiency is extremely poor in many cases, and although the method is effective in the case of drying a large amount of moisture contained in the entire recording medium after recording, in most cases practically no effect is demonstrated with respect to a wetting agent (solvent with a high boiling point) contained in the image. Further, in order to dry solvents with a high boiling point, heating at a high temperature of hundred and several tens of degrees is generally required, the specific temperature depending on the solvent type. Accordingly, damage to the recording media, such as paper yellowing and deformation and, in the worst case, inflammation cannot be ignored and such method is absolutely unsuitable for practical use.
In the past, products using the aforementioned technique to enhance drying on the usual paper in ink jet printers using aqueous inks have been marketed. However, in the case of ink jet recording, by contrast with electrophotography, a large amount of moisture contained in the ink is converted into water vapor that fills the inside of the apparatus, easily causing condensation and corrosion. Further, in recording on usual paper and special paper for ink jet recording, drying enhancement means becomes unnecessary. For these reasons, such products are presently not used, with the exception of very few commercial printers that require extremely high-speed recording on the usual paper.
Further, a problem arising when drying of an ink print is performed using a drying method represented by the method described in Patent Literature 7 is that a surface is locally melted and an image is burnt at special paper for ink jet printing.
Patent Literature 8 suggests a method for heating image-receiving paper containing a thermoplastic resin in the outer surface layer to make it transparent and smooth, thereby imparting gloss to the ink print, and also suggests that chemical substances that react under heating be added to the ink and recording media and the reaction be induced by heating after recording to improve the image hold-out. However, these suggestions relate to a special paper for ink jet recording that implies heat treatment after recording and are not applicable to general commercial-grade paper.
Thus, ink jet printing technology including inks and recording media that can be employed for ink jet printing and fixing on a commercial-grade paper with aqueous pigment inks and enable the use of the print immediately after the printing has not yet been realized.    (Patent Literature 1)    Japanese Patent Applications Laid-Open (JP-A) No. 2005-212327    (Patent Literature 2)    Japanese Patent Applications Laid-Open (JP-A) No. 11-078225    (Patent Literature 3)    Japanese Patent Applications Laid-Open (JP-A) No. 2001-347749    (Patent Literature 4)    Japanese Patent Applications Laid-Open (JP-A) No. 2007-144975    (Patent Literature 5)    Japanese Patent Applications Laid-Open (JP-A) No. 08-92513    (Patent Literature 6)    Japanese Patent (JP-B) No. 2860123    (Patent Literature 7)    Japanese Patent (JP-B) No. 2590822    (Patent. Literature 8)    Japanese Patent Applications Laid-Open (JP-A) No. 2004-209799